Alkyl isocyanates dimerization reactions

Reportedly, simple alkyl isocyanates do not dimerize upon standing. They trimerize to isocyanurates under comparable reaction conditions (57). Aliphatic isocyanate dimers can, however, be synthesized via the phosgenation of A[,A[-disubstituted ureas to yield /V-(ch1orocarhony1)ch1oroformamidine iatermediates which are subsequendy converted by partial hydrolysis and base catalyzed cycUzation. This is also the method of choice for the synthesis of l-alkyl-3-aryl-l,3-diazetidiones (mixed dimers of aromatic and aUphatic isocyanates) (58). 

N-Substituted amides can be prepared by direct attack of isocyanates on aromatic rings.The R group may be alkyl or aryl, but if the latter, dimers and trimers are also obtained. Isothiocyanates similarly give thioamides. The reaction has been carried out intramolecularly both with aralkyl isothiocyanates and acyl isothiocyanates.In the latter case, the product is easily hydrolyzable to a dicarboxylic acid this is a way of putting a carboxyl group on a ring ortho to one already there (34 is... 

The first examples of urea-containing dimeric capsules were discovered independently by Rebek and Bohmer [165,166]. These are based on calix[4]arene ethers, in which the lower rim ether units help to fix the calixarenes in a cone conformation via intramolecular interactions. Figure 60 shows the generic structure of such systems. Synthetically, the urea calixarenes are readily prepared. The calixarenes are first nitrated, followed by a reduction of these groups into amines. The urea derivatives are fixed to the upper rim by reaction of the p-amino calixarenes with isocyanates. Many systems were studied using differently substituted ureas which contain either short alkyl chains or simple phenyl derivatives. Studies also involved the changing of the lower rim ether substituents. 

The P-alkyl iminophosphoranes are considerably more reactive than the P-aryl compounds, but iminophosphoranes with alkoxy or chloro groups are also known to participate in the reaction. N-alkyl- or N-aryltrichloroiminophosphoranes are usually obtained as four membered ring dimers, which on thermolysis generate the monomers. For example, thermolysis of the dimeric N-methyltrichloroiminophosphorane 63 in o-dichlorobenzene in the presence of phenyl isocyanate affords N-methyl-N -phenylcarbodiimide 64 in 57 % yield. 

Catalysts which have been found to promote dimerization of phenyl isocyanate include pyridine (11), methylpyridine (12), triethylamine (13), X-methyl- (or ethyl-)morpholine, triethylphosphine, and other alkyl or alkyl-arylphosphines (14, 15). Alkylphosphines bring about a very violent polymerization since they act as active catalysts and the polymerization is quite exothermic. Triphenylphosphine is inactive. Alkyl-arylphosphines are not as active as alkylphosphines and permit better control of the reaction. Another convenient method (14, 16) for control of phosphine-catalyzed dimerization involves the addition of an alkylating agent such as benzyl chloride in an amount stoichioraetrically equivalent to the substituted phosphine present. Complete deactivation of the catalyst results. By this means the reaction may be mitigated or even quenched and then activated by the addition of more catalyst. 

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